This invention relates generally to an optical imaging system for soft x-ray or extreme ultraviolet (EUV) lithography, more specifically to a two-mirror optical imaging system with a two-reflection configuration.
Improvements of the cost and function of integrated circuits have largely depended on the ability to print smaller dimensions. In order to print smaller feature sizes, lithographic systems operate at shorter exposure wavelengths and use larger numerical apertures. This trends is expected to continue down to .lambda.=193 nm exposure for the production of 180 nm structures. Unfortunately, refractive optical systems are unlikely to operate at wavelengths much shorter than 193 nm, making reflective optical systems necessary. During the past several years, extreme ultraviolet lithography (EUVL, or soft x-ray projection lithography) has evolved from simple concept into a possible candidate for the mass production of future integrated circuits. The progress of EUVL was reviewed thoroughly in an article titled "EUV Lithography" by Hawryluk et al (Solid State Technology, July 1997, p151, and August 1997, p75).
In the optical system for EUV lithography, multiple-mirrors have been employed to successively reflect beams of light to produce images of distant objects. The EUV optical designs with best image quality are those with the largest number of mirrors. However, since the best reflector we can reasonably expect in EUV is 70% reflectivity and maybe 60%, each additional mirror reduces the energy reaching the wafer and lowers the throughput. In fact, the inherent loss in a design can render the design inoperable. Such limitation leads to the requirement of using more laser power, which dramatically increases the equipment cost and cost of wafer exposure.
In the hope of optimizing the optical imaging system for EUV lithography, many efforts have been made in the optical design. Various EUV imaging systems having a configuration of three- or four-mirrors have been proposed. These designs are represented in U.S. Pat. Nos. 5,805,365, 5,353,322, 5,315,629, 5,257,139, 5,220,590, and 5,063,586. Imaging systems having two mirrors but with four reflections are also described, such as in U.S. Pat. No. 5,212,588. Although mechanically simpler, the reduction of energy due to those reflections is as significant as in four-mirror systems. Most of these designs are only useful for a narrow slit ring-field. In these systems a large exposure field is achieved by scanning the mask and the wafer in the opposite directions at a different speed which is precise equal to the reduction ratio of the imaging system. Therefore it increases the cost of equipment, and the smaller instant ring-field decreases the energy throughput.
Kurihara et al has proposed a two-mirror system for EUV or soft x-ray reduction lithography, (J. Vac. Sci. Technol. B9, 1991, p3189) which is also a narrow slit ring-field system with a very narrow ring width. Various two-mirror Schwarzschild systems have also been demonstrated for EUV imaging, but these typically have a very small field-of-view (such as 25.times.50, .mu.m), which is too small to be useful for production (Tichenor et al, and Murakami et al, OSA Proceedings on EUV Lithography, Vol 23, 1994, p89 and p122).
It is well known that Lagrange invariant of optical system is the product of the beam solid angle (square of numerical aperture, NA.sup.2) and the image field area. The product is an invariant from radiation source, condenser, projection objective to image plane. The radiation power can be transmitted from radiation source to image is the product of Lagrange invariant and the brightness of source. Therefore it is desired that the Lagrange invariant of an optical system is as large as possible. The optical system for EUV lithography described in U.S. Pat. No. 5,805,365 has only a small Lagrange invariant with a field area of 25 mm.times.7 mm and NA 0.085. The Lagrange invariant of the systems in other prior arts is even smaller.
A need therefore exists in the art for an EUV lithographic projection system that uses an all reflective optical design yet provide high system throughput by achieving high efficiency of energy utilization. Such design will also lead to lower system cost and ease of fabrication.